A new basal archosauriform diapsid from the Lower Triassic of Argentina

EZCURRA, M.D.; LECUONA, A.; MARTINELLI, A.G.

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Año: 2010 vol. 30 p. 1433 - 1450

0272-4634

The best known South American Early Triassic archosauriform belongs to a putative proterosuchid briefly reported by José Bonaparte in 1981, coming from the Quebrada de los Fósiles Formation (Puesto Viejo Group, Argentina). This specimen consists of well-preserved natural external molds of a partial postcranium that preserves dorsal vertebrae, osteoderms, a dorsal rib, a possible gastralia, a chevron, a humerus, an ilium, two metapodials, and an ungual. We re-describe this specimen and identify autapomorphies that allow us to recognize Koilamasuchusgonzalezdiazi gen. et sp. nov. A cladistic analysis positions Koilamasuchus within Archosauriformes and more crownwards than Proterosuchus, Sarmatoscuhus, Fugusuchus, and Osmolskina, as the sister taxon of the clade that includes Erythrosuchidae and Archosauria. The presence of an iliac blade with a slightly convex dorsal margin and with a maximum length more than 3 times its maximum height places Koilamasuchus within Archosauriformes. Koilamasuchus is found as more derived than Proterosuchus because of the presence of an iliac preacetabular process, a pubic peduncle that forms an angle lowers than 45° to the longitudinal axis of the ilium, and dorsal body osteoderms. Koilamasuchus is more basal than erythrosuchids and more derived archosauriforms because of the presence of dorsal ribs with a poorly developed proximal end. The new taxon does not fit with the currently known lineages of non-archosaur archosauriforms, adding a new taxon which increases the diversity of the clade during the biotic recovery after the Permo-Triassic mass extinction.